The present invention relates to fiber optic ring sensing devices such as gyroscopes. This class of device operates by propagating a coherent, preferably polarized, light beam in an optical fiber which is coiled in a ring, .and sensing the shift in phase of the light traveling in the ring. The ring may provide a multi-pass path with an effective path length tens or hundreds of times longer than the fiber length, resulting in high sensitivity.
In a basic device of this sort, the phase shift of light in the ring is detected as the frequency difference between two beams travelling in opposite directions around the ring, e.g., via the beat frequency of a travelling beam and a commonly derived reference beam. Various beam modulating and signal processing techniques may be used to quantify the frequency shift. In any case, the ability to resolve this shift in an inertial sensing instrument is dependent on the initial use of a pure spectral source having a narrow line width.
The requirement for a narrow line width has generally required the use of gas or crystal laser sources, and has ruled out the use of laser diodes as sources for a fiber optic ring sensor instrument.
However, because laser diodes are extremely inexpensive, the development of a narrow line width laser diode system for a fiber optic sensor remains a desirable goal.